The increased sensitivity of cancer cells as compared to normal cells to increased temperature has been noted for some time. Cancer cells, because of their higher rate of metabolism, have higher resting temperatures compared to normal cells. The normal resting temperature of the cancer cell is known to be 37.5.degree. Centigrade, while that of the normal cell is 37.degree. Centigrade. Another physical characteristic that differentiates the cancer cells from the normal cells is that cancer cells die at lower temperatures than do normal cells. The temperature at which a normal cell will be killed and thereby irreversibly will be unable to perform normal cell functions is a temperature of 46.5.degree. Centigrade) on the average. The cancer cell, in contrast, will be killed at the lower temperature of 45.5..degree. Centigrade. The temperature elevation increment necessary to cause death in the cancer cell is determined to be at least approximately 8.0.degree. Centigrade, while the normal cell can withstand a temperature increase of at least 9.5.degree. Centigrade.
In attempts to solve the existing problem as to how to get energy into the cancer cells without affecting the normal cells, efforts have been made to couple electromagnetic fields extracellularly to tissues to attempt to induce heating. However, these efforts have not been effective largely because of the inability to differentiate the cancer cells form the normal cells.
The electromagnetic field interacts with tissue in several ways There are displacement currents due to the drift of electrons, polarization of atoms or molecules to produce dipoles and the interaction with dipoles already present. The coupling of electromagnetic energy to the tissue depends on the electrical conductivity (.sigma.) and the dielectric constant (.epsilon.). The power imparted to the tissue depends on the square of the amplitude of the field and the coupling constant to the tissue. The dielectric properties of the material depend on its composition and structure (i.e ions, polar molecules, etc.).
In general: EQU .epsilon.=.epsilon.'-j.epsilon."
where
.epsilon.'=real component related to energy stored in the material in electric fields
.epsilon..increment.=imaginary component related to loss in the form of heat= ##EQU1##
.sigma.=conductivity so that conductivity is related to the amount of heat loss Often as there is an increase in frequency, .epsilon.' decreases due to less ordering and .epsilon." increases.
In tissue a plot of the dielectric constant as a function of frequency often shows three dispersions. Each dispersion is related to a specific phenomenon. The .alpha. dispersion (at.apprxeq.80-100 Hz) is due to the interaction of the charges on the cell surface with the ions in solution and the impedence if the membrane system.
The .beta. dispersion (at.apprxeq.50 KHz) is related to the cell membrane's insulation of the H.sub.2 O. Above 10 GHz the .gamma. dispersion is due to the H.sub.2 O and electrolyte solution. To overcome these problems the common practice is to use a frequency&gt;&gt;1 KHz to short out the membrane effects and to deliver energy to the cytoplasm.
Consequently, frequencies greater than 1 KHz and usually greater than 1 MHz are utilized to overcome problems with the cell membrane and deliver energy to the cell. Traditionally, frequencies of 13 MHz or 2450 MHz are used. However, the problem remains that at these high frequencies not only are the cancer cells affected but the normal cells are also affected and consequently one is limited in the amount of energy which can be delivered to the cancer cells.
The present invention seeks to overcome this problem, by modifying the intracellular environment to allow the use of lower frequencies if possible and to enhance the effect in the cancer cells without affecting the normal cells.